[Python-Dev] PEP 563: Postponed Evaluation of Annotations

Guido van Rossum guido at python.org
Wed Nov 8 23:16:53 EST 2017

On Wed, Nov 8, 2017 at 5:49 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:

> On 8 November 2017 at 16:24, Guido van Rossum <guido at python.org> wrote:
> > I also don't like the idea that there's nothing you can do with a thunk
> > besides calling it -- you can't meaningfully introspect it (not without
> > building your own bytecode interpreter anyway).
> Wait, that wasn't what I was suggesting at all - with thunks exposing
> their code object the same way a function does (i.e. as a `__code__`
> attribute), the introspection functions in `dis` would still work on
> them, so you'd be able to look at things like which variable names
> they referenced, thus granting the caller complete control over *how*
> they resolved those variable names (by setting them in the local
> namespace passed to the call).

I understood that they would be translated to `lambda: <expr>`. It seems
you have a slightly more complex idea but if you're suggesting
introspection through dis, that's too complicated for my taste.

This is why they'd have interesting potential future use cases as
> general purpose callbacks - every local, nonlocal, global, and builtin
> name reference would implicitly be an optional parameter (or a
> required parameter if the name couldn't be resolved as a nonlocal,
> global, or builtin).

Yeah, but that's scope creep for PEP 563. Łukasz and I are interested in
gradually restricting the use of annotations to static typing with an
optional runtime component. We're not interested in adding different use
cases. (We're committed to backwards compatibility, but only until 4.0,
with a clear deprecation path.)

> > Using an AST instead of a string is also undesirable -- the AST changes
> in
> > each release, and the usual strong compatibility guarantees don't apply
> > here. And how are you going to do anything with it? If you've got a
> string
> > and you want an AST node, it's one call away. But if you've got an AST
> node
> > and you want either a string *or* the object to which that string would
> > evaluate, you've got a lot of work to do. Plus the AST takes up a lot
> more
> > space than the string, and we don't have a way to put an AST in a
> bytecode
> > file. (And as Inada-san pointed out a thunk *also* takes up more space
> than
> > a string.)
> >
> > Nick, please don't try to save the thunk proposal by carefully dissecting
> > every one of my objections. That will just prolong its demise.
> Just the one objection, since you seem to be rejecting something I
> didn't suggest (i.e. adding an opaque callable type that the dis and
> inspect modules didn't understand). I agree that would be a bad idea,
> but it's also not what I was suggesting we do.

I did not assume totally opaque -- but code objects are not very
introspection friendly (and they have no strong compatibility guarantees).

Instead, thunks would offer all the same introspection features as
> lambda expressions do, they'd just differ in the following ways:
> * the parameter list on their code objects would always be empty
> * the parameter list for their __call__ method would always be "ns=None"
> * they'd be compiled without CO_OPTIMIZED (the same as a class namespace)
> * they'd look up their closure references using LOAD_CLASSDEREF (the
> same as a class namespace)

I don't understand the __call__ with "ns-None" thing but I don't expect it

> That said, even without a full-fledged thunk based solution to
> handling lexical scoping I think there's a way to resolve the nested
> class problem in PEP 563 that works for both explicitly and implicitly
> quoted strings, while still leaving the door open to replacing
> implicitly quoted strings with thunks at a later date: stating that
> *if* users want such nested references to be resolvable at runtime,
> they need to inject a runtime reference to the outermost class into
> the inner class namespace.
> That is, if you want to take:
>     class C:
>         field = 1
>         class D:
>             def method(a: C.field):
>                 ...
> and move it inside a function, that would actually look like:
>     def f():
>         class C:
>             field = 1
>             class D:
>                 def method(a: C.field):
>                     ...
>         C.D.C = C # Make annotations work at runtime
>         return f
> That leaves the door open to a future PEP that proposes thunk-based
> annotations as part of proposing thunks as a new low level delayed
> evaluation primitive.

Sorry, that's not a door I'd like to leave open.

--Guido van Rossum (python.org/~guido)
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